This is a black mould that feeds on the honeydew excreted by aphids and other sap-sucking insects. It is unsightly and also blocks off light from the plant leaves that it covers. Apart from this, the fungus causes no problems, although it is usually associated with plants that are unhealthy as a result of the primary pests. Treat these pests as described under Aphids. The mould can easily be washed off with water.